FIELD OF THE INVENTION
The invention relates to a configuration for controlling a restraining device such as an airbag, seat belt pretensioning device, etc., and in particularly for use in a motor vehicle. Such a configuration includes an acceleration sensor for sensing acceleration and outputting an acceleration signal. First and second control units are provided for receiving and processing the acceleration signal. A firing circuit having a firing element for controlling a restraining device is also provided. The firing circuit has a first, a second and a third power switch controlled by the first and second control units. An energy store energizes the firing element when all three power switches are actuated.
A microcomputer that processes the acceleration signals of two acceleration sensors is described in U.S. Pat. No. 5,182,495. If an impact is detected, two power switches in the firing circuit are switched through by the microcomputer, so that energy from an energy source is applied to the firing element disposed between the power switches. Each power transistor in the firing circuit is connected to, in each case, one output of the microcomputer. The power transistors are configured here in such a way that the one power transistor requires a logic one at the output of the microcomputer to be switched on, and the other power transistor requires a logic zero. If the microcomputer chip is severely damaged owing to a high level of heat or a mechanical effect, it is highly probable that either all of its outputs will supply a logic one or all of its outputs will supply a logic zero. The anti-phase actuation of the power switches that is necessary for firing prevents a misfire when the microcomputer is fundamentally degraded.
Misfiring still takes place if the functionality of the microcomputer is faulty, for example if the processing routines for the acceleration signals are faulty.
Published, Non-Prosecuted, German Patent Application DE 40 16 644 A1 discloses a configuration in which a microcomputer evaluates the signal of an acceleration sensor and, where necessary, switches through an electrically controllable power switch in the firing circuit. A further acceleration signal of a further acceleration sensor is assessed in a control unit of analog construction. The control unit that is of analog construction switches on a second electrically controllable power switch in the firing circuit if it detects a minimum acceleration of the vehicle. A firing element is disposed between the two power switches, and energy is applied to it if both power switches are switched on.
Such a configuration has the disadvantage that, in the event of a serious malfunction or severe damage to the microcomputer, the first power switch is incorrectly closed and, owing to the threshold which is low in the analog circuit an acceleration signal with a low amplitude, caused for example by the vehicle travelling over a curb, is already sufficient to fire the firing element.
A generic type of configuration utilizing a microcomputer which processes the signals of two acceleration sensors is disclosed in U.S. Pat. No. 5,283,472. Via two of its outputs, the microcomputer controls two electrically controllable power switches of a firing circuit. In addition, the firing circuit contains a mechanical acceleration switch that closes under the effect of a minimum acceleration.
A disadvantage of this configuration is that currently available mechanical acceleration switches have long closing times, and are thus not suitable for detecting a side impact of a motor vehicle.